Method of Cooling Boil-Off Gas and Apparatus Therefor

ABSTRACT

There is provided a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream,
         wherein the SMR is provided in an SMR recirculating system comprising at least the steps of:   (a) compressing the SMR using at least one centrifugal compressor to provide a post-compression SMR stream;   (b) passing the post-compression SMR stream into the liquefaction heat exchanger system to cool the post-compression SMR stream and provide a cooled first SMR vapour stream;   (c) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system;   (d) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and a light SMR vapour stream;   (e) passing the light SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and   (f) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.

The present invention relates to a method of cooling a boil-off gas(BOG) stream from a liquefied gas tank, such as a cargo tank, such as ona floating vessel, using a single mixed refrigerant (SMR), and apparatustherefor. It is particularly, but not exclusively, a method for coolingBOG from a floating LNG storage tank.

Traditionally, boil-off gas from liquefied natural gas (LNG) storagetanks on board ships carrying LNG as a cargo (typically LNG carriers)has been used in the ship engines to provide power to the ship. Anyexcess BOG is then considered ‘waste gas’, and is typically sent to agas combustion unit (GCU), where it is disposed of by combustion.

However, ship engines have become increasingly more efficient, so thatless of the BOG is required for the engines. This means a greaterproportion of the BOG is sent to the GCU as waste gas. It is becomingeconomically attractive to reduce this loss of gas by re-liquefying itand returning it to the cargo tanks.

A conventional SMR cycle is shown in the accompanying FIG. 1. Boil-offgas from cargo tanks is compressed in a compressor (not shown) and sentfor cooling via pipeline 20. The compressed boil-off gas is first cooledin an aftercooler 14 using a readily available ambient cooling medium(e.g. seawater, freshwater, engine room cooling water, air), after whichit is cooled further in heat exchanger 12. This pre-cooled BOG is sentinto multi-stream (i.e. more than just two streams) heat exchanger 7(typically a brazed aluminium plate-fin heat exchanger), where it iscooled and condensed using an SMR recirculating system.

The heat exchanger 12 uses an external refrigerant (typically propane)supplied via pipeline 32, provided from a separate refrigerant cascade13.

In the SMR recirculating system, the mixed refrigerant gas fromrefrigerant receiver 1 flows through a pipeline 22 to a compressor 2.The SMR gas is compressed into pipeline 24. The gas in pipeline 24 issent into an aftercooler 6 which uses a readily available cooling medium(e.g. seawater, freshwater, engine room cooling water, air).

Downstream of the aftercooler 6, condensation of the refrigerant gas isperformed using heat exchange against a cold external refrigerant(typically propane) i n condenser 11. The cold temperatures of thisexternal refrigerant are created in the external refrigerant cascade 13.The refrigerant in pipeline 24 is at least partly condensed afterpassing through condenser 11, after which it enters a vapour-liquidseparator 8 to provide vapour and liquid phases.

The refrigerant liquid in pipeline 29 has its pressure reduced by flashvalve 9, leading to partial vaporisation and temperature reduction. Thepartially vaporised refrigerant liquid can then be sent into amulti-stream exchanger 7, where it is fully vaporised, thereby providingpartial cooling to the hot streams in the exchanger 7. Meanwhile, therefrigerant vapour in pipeline 26 is sent directly into exchanger 7,where it is cooled substantially. It leaves the exchanger 7, fully orpartly condensed, in pipeline 27, after which its pressure is reduced bya throttling valve 10 into pipeline 34 to its lowest temperature in theSMR recirculating system to achieve the required cooling in theexchanger 7. This provides the main cold stream for the exchanger 7.

The cold refrigerant in pipeline 34 is sent into exchanger 7, where itvaporises, cooling the hot streams. It merges with the depressurisedliquid sent from valve 9, and the combined refrigerant stream leavesexchanger 7 as a vapour via pipeline 28, to re-enter refrigerantreceiver 1.

Overall, the cooling duty for the re-liquefaction process in theconventional SMR cycle shown in FIG. 1 is provided by both the SMRrecirculating system and the external refrigerant cascade 13.

It is an object of the present invention to provide a simpler method,process and apparatus for cooling a BOG stream without an externalrefrigerant cascade, for applications requiring larger reliquefactioncapacities, such as Q-flex or Q-max LNG carriers with cargo capacitiestypically greater than 200,000 m³.

Thus, according to the first aspect of the present invention, there isprovided a method of cooling a boil-off gas (BOG) stream from aliquefied gas tank using a single mixed refrigerant (SMR) comprising atleast the step of heat exchanging the BOG stream with the SMR in aliquefaction heat exchanger system to provide a cooled BOG stream,

-   -   wherein the SMR is provided in an SMR recirculating system        comprising at least the steps of:

(a) compressing the SMR using at least one centrifugal compressor toprovide a post-compression SMR stream;

(b) passing the post-compression SMR stream into the liquefaction heatexchanger system to cool the post-compression SMR stream and provide acooled first SMR vapour stream;

(c) withdrawing the cooled first SMR vapour stream from the liquefactionheat exchanger system;

(d) separating the cooled first SMR vapour stream to provide aliquid-phase SMR stream and a light SMR vapour stream;

(e) passing the light SMR vapour stream through the liquefaction heatexchanger system to provide a condensed SMR stream; and

(f) expanding the condensed SMR stream to provide an expandedlowest-temperature SMR stream to pass through the liquefaction heatexchanger system for heat exchange against the BOG stream.

The present invention utilises one or more centrifugal compressorpackage(s), which are suitable for applications requiring largerreliquefaction capacities than can be achieved in a single oil-injectedscrew compressor. Centrifugal compressors have found widespread use inland-based natural gas liquefaction plants, which typically havecapacities much higher than that of off-shore reliquefaction. Unlikesmaller oil-injected screw compressors, centrifugal compressors have theadditional benefit of having effectively no oil carryover. The capacityof centrifugal compressors can provide a significantly higher flowcapacity, which renders a larger system employing centrifugalcompressors more economical overall.

SMR is a term in the art used to refer to a range of refrigerantsgenerally comprising a mixture of one or more hydrocarbons, inparticular usually methane, ethane and propane, and possibly also atleast butane, and nitrogen, optionally with one or more other possiblerefrigerants such as pentane. Various components and their ratios areknown for forming a particular SMR, and are not further describedherein.

Separating one or more of the streams as defined herein can be carriedout in any suitable separator, many of which are known in the art, andwhich are generally intended to provide at least one gaseous stream,typically a lighter stream available at or near an upper part of theseparator, and a heavier stream, typically comprising at least oneliquid phase, typically available at a lower end of the separator.

Expansion of a stream is possible through one or more suitable expansiondevices, generally including valves and the like.

The term “ambient cooling” as used herein relates to the use of anambient cooling medium, usually provided at an ambient temperature. Thisincludes seawater, freshwater, engine room cooling water, and air, andany combination thereof, which are typically easily available for use inproviding ambient cooling to a stream.

Optionally, the cooled first SMR vapour stream and/or the light SMRvapour stream are cooled against the expanded lowest-temperature SMRstream.

All liquefied gas tanks create or release boil-off gas for knownreasons, including tanks on liquefied gas carriers, barges and othervessels including transportation vessels. Liquefied gases can includethose having normal boiling points (at 1 atm) below 0° C., typically atleast below −40° C., such as various petroleum o r petrochemical gases,and including liquefied natural gas (LNG) having a normal boiling pointbelow −160° C.

Whilst BOG from liquefied gas tanks may be more readily usable onshore,it is especially desired to seek re-liquefaction of BOG offshore.However, space is typically limited offshore, especially on floatingvessels, and the ability to reduce the complexity of BOG re-liquefactioncan often achieve a reduction in the required CAPEX and plot arearequired.

Optionally, the BOG is from a liquefied cargo tank in a floating vessel,optionally an LNG cargo tank.

It is possible that the compression of the SMR in step (a) comprises theuse of more than one compressor, optionally in parallel or series orboth, to provide the post-compression SMR stream. The invention is notlimited by the method or type of compression of the SMR, other than theuse of at least one centrifugal screw compressor.

The liquefaction heat exchanger system may be any form of one or moreheat exchangers arranged in one or more units or stages, and able toallow heat exchange between two or more streams, and optionally havingat least one stream running countercurrently to one or more otherstreams in a part or portion of the system, in particular between theBOG stream and one of the refrigerant streams.

Where the liquefaction heat exchanger system comprises more than oneheat exchanger, the more than one heat exchangers may be in series or inparallel or a combination of in series and in parallel, and the morethan one heat exchangers may be separate or conjoined or contiguous,optionally in a single cooled unit or box, and optionally in the form ofone or more units or stages of providing the required heat exchange withthe BOG stream to liquefy the BOG stream.

The liquefaction heat exchanger system may comprise any suitablearrangement of two-stream or multi-stream heat exchangers arranged intoone or more connected sections, units or stages, optionally with onesection, unit or stage being ‘warmer’ than another section, unit orstage, in the sense of the average temperature therein.

Many liquefaction heat exchangers are known in the art which are able tobe part of or provide the liquefaction heat exchanger system, typicallycomprising plate-fin, shell & tube, plate & frame, shell & plate, coilwound, and printed circuit heat exchangers, or any combination thereof.

Optionally, the liquefaction heat exchanger system comprises amulti-unit liquefaction heat exchange comprising two multi-stream heatexchangers.

Alternatively, the liquefaction heat exchanger system comprises amulti-unit liquefaction heat exchange comprising one multi-stream heatexchanger and a plurality of two-stream heat exchangers.

Optionally, the liquefaction heat exchanger system in the presentinvention comprises one or more plate-fin heat exchangers.

Optionally, the liquefaction heat exchanger system in the presentinvention comprises a combination of one or more plate-fin heatexchangers and one or more two-stream plate-type (plate & frame or shell& plate) heat exchangers.

Heat exchangers generally have one or more entry points or ports foreach stream, and one or more exit points or ports for said stream, witha temperature gradient or gradient pathway thereinbetween. Most streamspassing through a heat exchanger pass typically through ‘all’ the heatexchanger, that is from an entry point or port at one end or side of theheat exchanger to an exit point or port, optionally at another end orside but not limited thereto, so as to achieve the maximum heat exchangepossible between the entry and exit, i.e. the maximum temperature changeor phase change possible along the temperature gradient pathway. Suchstreams have ‘fully’ or ‘wholly’ passed through the heat exchanger.

Some streams may only pass through a partial portion or amount of a heatexchanger, generally by either having an entry point or port at anintermediate temperature or location along the maximum possibletemperature gradient pathway, or by having an exit point or port at anintermediate temperature along the temperature gradient pathway, orboth. Such streams have passed through only part of the heat exchanger.

In the present invention, the liquefaction heat exchange can be providedin a single stage or in a multi-stage arrangement, optionally in linewith the number of liquefaction heat exchangers in the liquefaction heatexchanger system, but not limited thereto where more than one heatexchange stage can be provided with a single liquefaction heatexchanger.

Optionally, the liquefaction heat exchanger system is a singleliquefaction heat exchanger. In one further option, the method comprisespassing the light SMR vapour stream partly through the singleliquefaction heat exchanger prior to step (f), i.e. passing the lightSMR vapour stream into the single liquefaction heat exchanger at anintermediate temperature along the heat exchange.

In another further option, the method comprises passing the light SMRvapour stream fully through the single liquefaction heat exchanger priorto step (f).

Optionally, where the liquefaction heat exchanger system is a singleliquefaction heat exchanger, withdrawal of the cooled first SMR vapourstream from the liquefaction heat exchanger system in step (c) can occurat an intermediate temperature along the heat exchange occurring in theheat exchanger, optionally at a temperature that is similar to the entryfor the light SMR vapour stream into the liquefaction heat exchangersystem to provide a condensed SMR stream.

Thus, optionally, step (c) of the present invention compriseswithdrawing the cooled first SMR vapour stream from the liquefactionheat exchanger system prior to the coolest part of the liquefaction heatexchanger system, i.e. achieving partial passageway through theliquefaction heat exchanger system.

The light SMR vapour stream may be passed (back) into the liquefactionheat exchanger system at a temperature that is higher than, lower than,the same as, or similar to, the temperature of the withdrawn cooledfirst SMR vapour stream of step (c).

Optionally, the light SMR vapour stream passes into the liquefactionheat exchanger system at a temperature that is similar to thetemperature of the withdrawn cooled first SMR vapour stream of step (c).

Alternatively, the liquefaction heat exchanger system may be amulti-unit liquefaction heat exchange or exchanger comprising two,optionally more than two, units, and the expanded lowest-temperature SMRstream passes through each unit.

Where the liquefaction heat exchange is provided by more than oneliquefaction heat exchanger units and/or stages, optionally the cooledfirst SMR vapour stream passes into a first unit and/or stage, and thelight SMR vapour stream passes into a second unit and/or stage.Alternatively optionally, the cooled first SMR vapour stream passes intoa first heat exchange unit, and the light SMR vapour stream passes intoboth a first heat exchange unit and a second heat exchanger unit.

Where the liquefaction heat exchange is provided by more than oneliquefaction heat exchanger units and/or stages, also optionally thefirst or warmer stage comprises either a multi-stream heat exchangersuch as a plate-fin heat exchanger, or a series of distinct heatexchangers, optionally in series, in parallel, or both, at least one ofwhich is able to cool the post-compression SMR stream and provide acooled first SMR vapour stream prior to separating the cooled first SMRvapour stream to provide a liquid-phase SMR stream and a light SMRvapour stream.

Optionally, the method of the present invention further comprises thesteps of expanding the liquid-phase SMR stream of step (d), and passingthe expanded liquid-phase SMR stream into the liquefaction heatexchanger system.

Optionally, the method of the present invention further comprises thestep of combining the expanded liquid-phase SMR stream with the expandedlowest-temperature SMR stream in the liquefaction heat exchanger system,further optionally, between two stages or units of a multi-stage ormulti-unit liquefaction heat exchanger system.

Optionally, the method of the present invention alternatively furthercomprises the step of combining the expanded liquid-phase SMR streamwith the expanded lowest-temperature SMR stream after the liquefactionheat exchanger system.

The method of the present invention provides a post-liquefaction heatexchange SMR stream, or a post-cooling vapour SMR stream, forrecirculation or reuse as part of the SMR recirculating system. Thispost stream is optionally the expanded liquid-phase SMR stream combinedwith the expanded lowest-temperature SMR stream, being combined eitherwithin or after the liquefaction heat exchanger system.

Thus, optionally, the method of the present invention further comprisesrecycling the expanded lowest-temperature SMR stream after theliquefaction heat exchanger for providing the SMR, typically with theadditional expanded liquid-phase SMR stream.

In the present invention, it is intended that the post-compression SMRstream of step (a) does not undergo any external refrigerant coolingprior to step (d), such that an external refrigerant cascade is notrequired. The SMR liquefaction heat exchanger system itself wholly orsubstantially provides the refrigerant cooling required to condense thelight SMR vapour stream prior to its expansion back into theliquefaction heat exchanger system.

Optionally, the BOG stream also does not undergo any externalrefrigerant cooling prior to passing through the liquefaction heatexchanger.

In this way, the expanded lowest-temperature SMR stream provides thecooling of the post-compression SMR stream, and preferably, the expandedlowest-temperature SMR stream provides all the sub-ambient refrigerantcooling duty for cooling the BOG stream and in the SMR recirculatingsystem.

According to another aspect of the present invention, there is providedan SMR recirculating system for use with a method of cooling a boil-offgas (BOG) stream from a liquefied gas tank using a single mixedrefrigerant (SMR) comprising at least the step of heat exchanging theBOG stream with the SMR in a liquefaction heat exchanger system toprovide a cooled BOG stream,

-   -   wherein the SMR is provided in an SMR recirculating system        comprising at least the steps of:

(a) compressing the SMR using at least one centrifugal compressor toprovide a post-compression SMR stream;

(b) passing the post-compression SMR stream into the liquefaction heatexchanger system to cool the post-compression SMR stream and provide acooled first SMR vapour stream;

(c) withdrawing the cooled first SMR vapour stream from the liquefactionheat exchanger system;

(d) separating the cooled first SMR vapour stream to provide aliquid-phase SMR stream and a light SMR vapour stream;

(e) passing the light SMR vapour stream through the liquefaction heatexchanger system to provide a condensed SMR stream; and

(f) expanding the condensed SMR stream to provide an expandedlowest-temperature SMR stream to pass through the liquefaction heatexchanger system for heat exchange against the BOG stream.

Optionally, the SMR recirculating system is for use in cooling BOG froma liquefied cargo tank in a floating vessel, optionally an LNG cargotank.

Optionally, the SMR recirculating system is for use with a liquefactionheat exchanger system as defined herein.

Optionally, the SMR recirculating system further comprises one or morefurther steps as herein described in relation to the method of cooling aBOG stream.

It is intended that the SMR recirculating system of the presentinvention is able to provide all the sub-ambient refrigerant coolingduty for cooling a boil-off gas stream from a liquefied gas tank and inthe SMR recirculating system.

According to another aspect of the present invention, there is providedan apparatus for cooling a boil-off gas (BOG) stream from a liquefiedgas tank comprising a single mixed refrigerant (SMR) recirculatingsystem as defined herein and a liquefaction heat exchanger for heatexchange against the BOG stream.

According to a further aspect of the invention, there is provided amethod of integratively designing a vessel having a method of cooling aboil-off gas (BOG) stream from a liquefied gas tank using a single mixedrefrigerant (SMR) comprising at least the step of heat exchanging theBOG stream with the SMR in a liquefaction heat exchanger system toprovide a cooled BOG stream, wherein the SMR is provided in an SMRrecirculating system comprising at least the steps of:

(a) compressing the SMR using at least one centrifugal compressor toprovide a post-compression SMR stream;

(b) passing the post-compression SMR stream into the liquefaction heatexchanger system to cool the post-compression SMR stream and provide acooled first SMR vapour stream;

(c) withdrawing the cooled first SMR vapour stream from the liquefactionheat exchanger system;

(d) separating the cooled first SMR vapour stream to provide aliquid-phase SMR stream and a light SMR vapour stream;

(e) passing the light SMR vapour stream through the liquefaction heatexchanger system to provide a condensed SMR stream; and

(f) expanding the condensed SMR stream to provide an expandedlowest-temperature SMR stream to pass through the liquefaction heatexchanger system for heat exchange against the BOG stream.

According to a further aspect of the invention, there is provided amethod of integratively designing an SMR recirculating system for usewith a method of cooling a boil-off gas (BOG) stream from a liquefiedgas tank comprising the same or similar steps as described herein.

According to a still further aspect of the invention, there is provideda method of designing a process for cooling a boil-off gas (BOG) streamfrom a liquefied gas tank using a single mixed refrigerant (SMR)comprising the same or similar steps as described herein.

According to a still further aspect of the invention, there is provideda method of designing an SMR recirculating system for use with a methodof cooling a boil-off gas (BOG) stream from a liquefied gas tankcomprising the same or similar steps as described herein.

The designing methods as discussed herein may incorporate computer aidedprocesses for incorporating the relevant operational equipment andcontrols into the overall vessel construction and may incorporaterelevant cost, capacity of operation parameters into the methodology anddesign. The methods described herein may be encoded onto media that issuitable for being read and processed on a computer. For example, codeto carry out the methods described herein may be encoded onto a magneticor optical media which can be read by and copied to a personal ormainframe computer. The methods may then be carried out by a designengineer using such a personal or mainframe computer.

Embodiments and an example of the present invention will now bedescribed by way of example only and with reference to the accompanyingschematic drawings in which:

FIG. 1 is a schematic view of a prior art method of cooling a BOG streamusing a prior art SMR system;

FIG. 2 is a schematic view of a method of cooling a BOG stream using anSMR system according to a general embodiment of the present invention;

FIG. 3 is a schematic view of a method of cooling a BOG stream using anSMR system according to a first embodiment of the present invention;

FIG. 4 is a schematic view of a method of cooling a BOG stream using anSMR system according to a second embodiment of the present invention;

FIG. 5 is a schematic view of a method of cooling a BOG stream using anSMR system according to a third embodiment of the present invention;

FIG. 6 is a schematic view of a method of cooling a BOG stream using anSMR system according to a fourth embodiment of the present invention;

FIG. 7 is a schematic view of a method of cooling a BOG stream using anSMR system according to a fifth embodiment of the present invention;

FIG. 8 is a schematic view of a method of cooling a BOG stream using anSMR system according to a sixth embodiment of the present invention; and

FIG. 9 is a schematic view of a method of cooling a BOG stream using anSMR system according to a seventh embodiment of the present invention

Where relevant, the same reference numerals are used in differentFigures to represent the same or similar feature.

FIG. 1 is a prior art arrangement described hereinabove, which requiresan external refrigerant circuit and apparatus based on cascade 13 toachieve re-liquefaction of the compressed BOG using an SMR recirculatingsystem and a compressor 2.

FIG. 2 shows a method of cooling a boil-off gas stream from a liquefiedgas tank according to a general embodiment of the present invention,using a single mixed refrigerant (SMR), and comprising at least the stepof heat exchanging the BOG stream with the SMR in a liquefaction heatexchanger system to provide a cooled BOG stream, and wherein the SMR isprovided in an SMR recirculating system according to another embodimentof the present invention.

In more detail, FIG. 2 shows a BOG stream 70 provided from one or moreLNG cargo tanks (not shown) and already compressed in a compressor (alsonot shown). The BOG stream 70 is optionally ambient cooled in a firstambient heat exchanger 60, using a readily available cooling medium(e.g. seawater, freshwater, engine room cooling water, air). Thisoptionally cooled (and compressed) BOG stream 71 is then passed into aliquefaction heat exchanger system 40.

The liquefaction heat exchanger system 40 may comprise any form orarrangement of one or more heat exchangers able to allow heat exchangebetween two or more streams, optionally between multiple streams, andoptionally having at least one stream running countercurrently to one ormore other streams in a part or portion of the system, in particularbetween the BOG stream and one of the refrigerant. Any arrangement ofmore than one heat exchanger may be in series or in parallel or acombination of in series and in parallel, and the heat exchangers may beseparate or conjoined or contiguous, optionally in a single cooled unitor box, and optionally in the form of one or more stages of providingthe required heat exchange with the BOG stream to liquefy the BOGstream.

Liquefaction heat exchanger systems comprising more than one heatexchanger generally have one section, unit or stage being ‘warmer’ thananother section, unit or stage, in the sense of the average temperaturetherein.

Some variants of suitable liquefaction heat exchanger systems arediscussed and shown hereinafter. The skilled person can recognise othervariants, and the invention is not limited thereby.

In the general liquefaction heat exchanger system 40 shown in FIG. 2,the cooled (and compressed) BOG stream 71 is condensed by colder streamsdiscussed hereinafter, generated in the SMR recirculating system 200.The condensed BOG stream leaves the exchanger system 40 via pipeline 73,and can be returned back to the LNG cargo tanks.

In the SMR system 200, an initial stream of SMR refrigerant gas 74 froma refrigerant receiver 51 is sent to a centrifugal compressor 52.Centrifugal compressors are well known in the art, and not furtherdescribed herein. Centrifugal compressors are well proven in industryand are cost effective, especially for larger scale or larger volumecompression.

In FIG. 2, compressing the initial SMR stream 74 using the onecentrifugal compressor package 52 provides a post-compression SMR stream79. The centrifugal compressor package may consist of multiple stages ofcompression, optionally with intercoolers using a readily availablecooling medium (e.g. seawater, freshwater, engine room cooling water,air) between some or all of the compression stages.

The post-compression SMR stream 79 is cooled in a second ambient heatexchanger 56 using a readily available cooling medium (e.g. seawater,freshwater, engine room cooling water, air) to provide a cooler firstvapour stream 80. Depending on the composition and pressure of therefrigerant, as well as on the temperature achieved in the secondambient heat exchanger 56, some condensation of the SMR may start tooccur.

The cooler first vapour stream 80 passes into the liquefaction heatexchanger system 40, where the refrigerant is cooled and at leastpartially condensed. The cooled first SMR vapour stream 81 is withdrawnfrom an intermediate temperature along the liquefaction heat exchangersystem 40, and enters a vapour-liquid separator 58. In the separator 58,a liquid-phase SMR stream 82 can be drained via pipeline 82.

Thereafter, the pressure of the liquid-phase SMR stream 82 can bereduced by a flash valve 59, resulting in some vaporisation and anassociated reduction in temperature. The expanded, or at least partlyvaporised, liquid-phase SMR stream 83 can be sent into the heatexchanger system 40, where it provides some cooling to warmer streams,while itself being vaporised.

In the separator 58, a light SMR vapour stream 84 is also sent into theheat exchanger system 40. In FIG. 2, the light SMR vapour stream 84enters the heat exchanger system 40 at an intermediate temperature,optionally at a similar temperature to that at the withdrawal of thecooled first SMR vapour stream 81. In the heat exchanger system 40, thislight SMR vapour stream 84 is cooled until it partly or whollycondenses, leaving the heat exchanger system 40 as a condensed SMRstream 85. Thereafter, the pressure is reduced via throttling valve 61,leading to partial vaporisation and temperature reduction to provide theexpanded lowest-temperature SMR stream 86. The expandedlowest-temperature SMR stream 86 is the coolest SMR refrigerant streamin the SMR system 200.

The expanded lowest-temperature SMR stream 86 is sent back into heatexchanger system 40, where it vaporises as it heats up, and in doing so,cools the warmer streams in the heat exchanger system 40 to provide themajority of the cooling duty. The SMR refrigerant stream 86 can mergewith the expanded liquid-phase SMR stream 83 to form a single streamwhich leaves the heat exchanger system 40 as a post-cooling vapourstream 89, to be returned to refrigerant receiver 51.

In this way, the requirement in prior art arrangement in FIG. 1 for anexternal refrigerant cascade is removed. This represents a reduction incapital expenditure, and in overall plant size. The partial condensationof SMR vapour is achieved without an external refrigerant cascade loop,having shifted that duty to the SMR recirculating system only.

FIG. 3 shows a more detailed SMR recirculating system 101 being a firstvariation example of the SMR recirculating system 200 shown in FIG. 2.The first SMR recirculating system 101 comprises a single multi-streamliquefaction heat exchanger 57, (typically a brazed aluminium plate-finheat exchanger), where the cooled (and compressed) BOG stream 71 iscondensed by the colder streams discussed herein before in the SMRrecirculating system 200.

FIG. 4 shows a second variation example SMR recirculating system 102 ofthe SMR recirculating system 200 shown in FIG. 2, wherein theliquefaction heat exchanger system now comprises two heat exchangers,being the first and second multi-stream heat exchange units 64 and 62.In FIG. 4, there is a mixing of cold streams externally of the heatexchange units 64 and 62. That is, the expanded lowest-temperature SMRstream or coldest refrigerant stream 86 is sent into the second unit 62,where it starts to vaporise as it heats up, and in doing so, cools thewarmer streams in the second unit 62, and then exits as a part-warmerSMR stream 87 prior to merging with the expanded liquid-phase SMR stream83 to form a combined stream 88, which then passes into the first unit64 to cool the warmer streams in the first unit 64, and leaving thefirst unit 64 as a post-cooling vapour stream 89, to be returned torefrigerant receiver 51. Meanwhile, the cooled BOG from the first unit64 passes as stream 72 into the second cooler unit 62.

The first and second heat exchange units 64 and 62 may be contiguous orseparate.

FIG. 5 shows a third variation example SMR recirculating system 103,being a further variation of the SMR recirculating system 102 shown inFIG. 4. In FIG. 5, the liquefaction heat exchanger system comprisesfirst and second multi-stream heat exchange units 63 and 62. Comparedwith FIG. 4, the expanded liquid-phase SMR stream 83 and part-warmer SMRstream 88 are kept separate in first unit 63. The first and secondwarmer SMR streams 90 and 91 provided by the liquefaction heat exchangersystem are combined in the vapour phase after they leave the first unit63 to form a combined post-cooling vapour stream 89, to be returned torefrigerant receiver 51.

FIG. 6 shows a fourth variation example SMR recirculating system 104,being another variation of the SMR recirculating system 102 shown inFIG. 4. In FIG. 6, the liquefaction heat exchanger system comprisesfirst and second multi-stream heat exchange units 63A and 62. Comparedwith FIG. 4, the light SMR vapour stream 95 provided by thevapour-liquid separator 58 now passes into the warmer first unit 63A toprovide an intermediate stream 92, prior to passage through the coolersecond unit 62 (to exit as a condensed SMR stream 85).

FIG. 7 shows a fifth variation example SMR recirculating system 105,being a combination of the third SMR recirculating system 103 shown inFIG. 5 and the fourth SMR recirculating system 104 shown in FIG. 6. InFIG. 7, the liquefaction heat exchanger system comprises first andsecond multi-stream heat exchange units 65 and 62, and the light SMRvapour stream 95 provided by the vapour-liquid separator 58 now passesinto the first warmer unit 65 (to provide an intermediate stream 92,prior to passage through the second cooler unit 62 to exit as acondensed SMR stream 85), and the expanded liquid-phase SMR stream 83and part-warmer SMR stream 88 are kept separate in first unit 65. Thefirst and second warmer SMR streams 93 and 94 provided by theliquefaction heat exchanger system are combined in the vapour phaseafter they leave the first unit 65 to form a combined post-coolingvapour stream 89, to be returned to refrigerant receiver 51.

FIG. 8 shows a sixth variation example SMR recirculating system 106,being a combination of the first SMR recirculating system 101 shown inFIG. 3 and the fourth SMR recirculating system 104 shown in FIG. 6. InFIG. 8, the liquefaction heat exchanger system comprises a singlemulti-stream liquefaction heat exchanger 66, and the light SMR vapourstream 95 provided by the vapour-liquid separator 58 now passes fullythrough the heat exchanger 66 (to provide a condensed SMR stream 85),whilst the expanded liquid-phase SMR stream 83 merges with therefrigerant stream 86 at an intermediate location within the heatexchanger 66 to form a single stream which leaves the heat exchanger 66as a post-cooling vapour stream 89, to be returned to refrigerantreceiver 51.

FIG. 9 shows a seventh SMR variation example recirculation system 107,being a variant of the SMR recirculating system 104 shown in FIG. 6,wherein the first multi-stream heat exchange unit 63A in theliquefaction heat exchanger system is replaced by a series of two-streamheat exchangers. The series of two-stream heat exchangers still providethe same first and warmer stage or section of the liquefaction heatexchanger system, now using a series of distinct heat exchangerssuitably arranged to work together.

In FIG. 9, the cooler first vapour stream 80 passes into a firsttwo-stream heat exchanger 96 against a stream discussed hereinafter, toprovide the cooled first SMR vapour stream 81 in the same manner asbefore, to pass into the vapour-liquid separator 58. From the separator58, a liquid-phase SMR stream 82 is expanded by a flash valve 59 toprovide an at least partly vaporised, liquid-phase SMR stream 83. Theseparator 58 also provides the light SMR vapour stream 95, which passesinto a second two-stream heat exchanger 97 to provide an intermediatestream 92 prior to its passage into the same second unit 62 as discussedand shown in FIG. 6.

Meanwhile, the cooled and compressed BOG stream 71 passes into a thirdtwo-stream heat exchanger 98 to provide a cooler BOG stream 72 to passinto the second cooler unit 62.

The second unit 62 in FIG. 9 provides the condensed BOG stream 73 in thesame manner as described above, and a part-warmer SMR stream 87, whichmerges with the expanded liquid-phase SMR stream 83 to form a combinedstream 88, which is then divided into part-streams 99A and 99B.Part-stream 99A passes into the second heat exchanger 97, andpart-stream 99B passes into the third heat exchanger 98. Their exitstreams combine to form a combined stream 100 which then passes into thefirst heat exchanger 96 to exit as the post-cooling vapour stream 89.

Where the liquefaction heat exchanger system comprises multiple heatexchanger units, the present invention is not limited by the relativepositioning of the first and second units, which may be contiguous orseparate.

It is possible that the composition and/or ratio of components in theSMR can be varied to achieve best effect for each arrangement of thepresent invention. It is also possible that the SMR composition isdifferent in each of the examples shown in FIGS. 3-9.

The present invention is a modification of a typical single mixedrefrigerant (SMR) cycle for LNG re-liquefaction in particular, thatallows the use of a centrifugal compressor in the mixed refrigerantsystem, without the requirement of an external refrigerant cascade. Incomparison with the typical arrangement, the present innovation allowsfor reduced complexity, fewer pieces of equipment, reduced capital cost,and is suitable for applications requiring larger reliquefactioncapacities.

1. A method of cooling a boil-off gas (BOG) stream from a liquefied gastank using a single mixed refrigerant (SMR) comprising the steps of:heat exchanging the BOG stream with the SMR in a liquefaction heatexchanger system to provide a cooled BOG stream, wherein the SMR isprovided in an SMR recirculating system comprising the steps of (a)compressing the SMR using at least one centrifugal compressor to providea post-compression SMR stream; (b) passing the post-compression SMRstream into the liquefaction heat exchanger system to cool thepost-compression SMR stream and provide a cooled first SMR vapourstream; (c) withdrawing the cooled first SMR vapour stream from theliquefaction heat exchanger system; (d) separating the cooled first SMRvapour stream to provide a liquid-phase SMR stream and a light SMRvapour stream; (e) passing the light SMR vapour stream through theliquefaction heat exchanger system to provide a condensed SMR stream;and (f) expanding the condensed SMR stream to provide an expandedlowest-temperature SMR stream to pass through the liquefaction heatexchanger system for heat exchange against the BOG stream.
 2. The methodas claimed in claim 1 wherein the BOG is from a liquefied cargo tank ina floating vessel.
 3. The method as claimed in claim 1 wherein theliquefaction heat exchanger system comprises a single liquefaction heatexchanger.
 4. The method as claimed in claim 1 further comprising instep (e) the step of passing the light SMR vapour stream partly througha single liquefaction heat exchanger.
 5. The method as claimed in claim1 further comprising in step (e) the step of passing the light SMRvapour stream fully through a single liquefaction heat exchanger.
 6. Themethod as claimed in claim 1 wherein the liquefaction heat exchangersystem comprises a multi-unit liquefaction heat exchange comprising atleast two heat exchange units, and the BOG stream and the expandedlowest-temperature SMR stream pass through each of the at least two heatexchange units.
 7. The method as claimed in claim 1 further comprisingthe steps of passing the post-compression SMR stream into a first heatexchange unit of the at least two heat exchange units, and passing thelight SMR vapour stream into a second heat exchanger unit of the atleast two heat exchange units.
 8. The method as claimed in claim 1further comprising the steps of passing the post-compression SMR streaminto a first heat exchange unit, and passing the light SMR vapour streaminto both the first heat exchange unit and a second heat exchange unit.9. The method as claimed in claim 1 wherein the liquefaction heatexchanger system comprises a multi-unit liquefaction heat exchangecomprising two multi-stream heat exchangers.
 10. The method as claimedin claim 1 wherein the liquefaction heat exchanger system comprises amulti-unit liquefaction heat exchange comprising one multi-stream heatexchanger and a plurality of two-stream heat exchangers.
 11. The methodas claimed in claim 1 further comprising the step of ambient-cooling thepost-compression SMR stream prior to step (b).
 12. The method as claimedin claim 1 further comprising the steps of expanding the liquid-phaseSMR stream, and passing the expanded liquid-phase SMR stream into theliquefaction heat exchanger system.
 13. The method as claimed in claim 1further comprising the step of combining the expanded liquid-phase SMRstream with the expanded lowest-temperature SMR stream in theliquefaction heat exchanger system.
 14. The method as claimed in claim 1wherein the liquefaction heat exchanger system comprises a multi-unitliquefaction heat exchanger system, and further comprising the step ofcombining the expanded liquid-phase SMR stream with the expandedlowest-temperature SMR stream between two units of the multi-unitliquefaction heat exchanger system.
 15. The method as claimed in claim 1further comprising the step of combining the expanded liquid-phase SMRstream with the expanded lowest-temperature SMR stream after theliquefaction heat exchanger system
 16. The method as claimed in claim 1wherein step (f) provides a post-cooling vapour SMR stream forrecirculation or reuse as part of the SMR recirculating system. 17.(canceled)
 18. The method as claimed in claim 1 wherein thepost-compression SMR stream of step (a) does not undergo any externalrefrigerant cooling prior to step (d).
 19. The method as claimed inclaim 1 wherein the BOG stream does not undergo any external refrigerantcooling prior to passing through the liquefaction heat exchanger. 20.The method as claimed in claim 1 wherein the liquefaction heat exchangersystem comprises one or more plate-fin heat exchangers.
 21. The methodas claimed in claim 1 wherein the expanded lowest-temperature SMR streamprovides the cooling of the first SMR vapour stream.
 22. A recirculatingsystem for use with a method of cooling a boil-off gas (BOG) stream froma liquefied gas tank using a single mixed refrigerant (SMR), the systemcomprising: a liquefaction heat exchanger system for heat exchanging theBOG stream with the SMR to provide a cooled BOG stream, and an SMRrecirculating system comprising the steps of: (a) compressing the SMRusing at least one centrifugal compressor to provide a post-compressionSMR stream; (b) passing the post-compression SMR stream into theliquefaction heat exchanger system to cool the post-compression SMRstream and provide a cooled first SMR vapour stream; (c) withdrawing thecooled first SMR vapour stream from the liquefaction heat exchangersystem; (d) separating the cooled first SMR vapour stream to provide aliquid-phase SMR stream and a light SMR vapour stream; (e) passing thelight SMR vapour stream through the liquefaction heat exchanger systemto provide a condensed SMR stream; and (f) expanding the condensed SMRstream to provide an expanded lowest-temperature SMR stream to passthrough the liquefaction heat exchanger system for heat exchange againstthe BOG stream.
 23. The recirculating system as claimed in claim 22 foruse with cooling BOG from a liquefied gas cargo tank in a floating LNGcargo tank. 24-26. (canceled)
 27. An apparatus for cooling a boil-offgas (BOG) stream from a liquefied gas tank comprising: a single mixedrefrigerant (SMR) recirculating system as defined in claim 22 and aliquefaction heat exchanger system for heat exchange against the BOGstream.